Lactation and weaning effects on physiological and behavioral response to stressors.

Two experiments tested the effects of lactation and weaning on heart rate (HR), corticosterone, and behavioral responses to stress in Wistar rats. In Experiment 1, HRs in lactating, weaning, and control animals were recorded for 10-min periods before, during, and after immobilization stress. Compared with control animals, lactating and weaning animals showed a diminished initial HR response. In addition, HRs of weaning animals failed to habituate and showed a delayed decline after stress termination. In Experiment 2, behaviors, HRs, and corticosterone levels in the elevated plus maze (EPM) were compared among lactating, weaning, and control animals. Compared with control animals, weaning animals exhibited more anxiety behaviors. Contrary to expectation, compared to the other two groups, lactating animals exhibited more closed-arm entries, although they may have been motivated by maternal behavior, rather than anxiety. Initial HR responses to the plus maze were attenuated in lactating animals. Corticosterone levels after the plus maze were lowest in the lactating dams and highest in the control animals. The results from these two experiments are consistent with effects of breast-feeding and weaning observed in humans. In general, lactation is associated with an attenuated initial HR response to stress, while weaning is associated with exacerbated response to stressors.